The above is all that is needed for the physics engine. ConvertUnits is used to make the border the size of the window but in physics units. You need to keep your physics world in meters and your rendering can be any scale you like.

Am I right if I say that the float value of 0.75 in line "float halfWidth = ConvertUnits.ToSimUnits(viewport.Width)
/ 2f - 0.75f;" is the borders thickness in simulation units?

That means, that "float halfWidht = ConvertUnits.ToSimUnits (viewport.Width) / 2f" figures out the horizontal center of my viewport in simulation units (same with halfHeight). This is followed by the subtaction of the border-thickness in simulation
units. In my case 10px are 1m. This follows in a border thickness of 7.5px (~8px). Am I right with this situation?

In addition to this could someone give me a hint what the vertice stuff for rendering does? I do not unterstand why some Vector2 by 5.25f or halfWidth/halfHeight is increased/decreased with the value 2...